Multi-functional separator for a lithium secondary battery and manufacturing method

ABSTRACT

A lithium secondary battery comprising a cathode, an anode, an elastic and flame retardant composite separator disposed between the cathode and the anode, and a working electrolyte. The elastic flame retardant composite separator comprises a high-elasticity polymer and from 1% to 99% by weight of a flame retardant additive dissolved in, dispersed in, or bonded to the high-elasticity polymer. The composite separator has a thickness from 50 nm to 100 μm and a lithium ion conductivity from 10−8 S/cm to 5×10−2 S/cm at room temperature and the high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% when measured without any additive dispersed therein. The polymer composite may further comprise particles of an optional inorganic solid electrolyte dispersed therein.

FIELD

The present disclosure relates to the field of rechargeable lithium battery, including the lithium-ion battery and the lithium metal battery (a secondary battery that makes use of lithium metal as an anode active material) and a method of manufacturing same.

BACKGROUND

Lithium-ion and lithium (Li) metal cells (including Lithium-sulfur cell, Li-air cell, etc.) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium metal has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound (except Li_(4.4)Si) as an anode active material. Hence, in general, rechargeable Li metal batteries have a significantly higher energy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having high specific capacities, such as TiS₂, MoS₂, MnO₂, CoO₂ and V₂O₅, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were dissolved from the lithium metal anode and transferred to the cathode through the electrolyte and, thus, the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.

Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. These issues are primarily due to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway. Many attempts have been made to address the dendrite-related issues, as briefly summarized below:

Fauteux, et al. [D. Fauteux, et al., “Secondary Electrolytic Cell and Electrolytic Process,” U.S. Pat. No. 5,434,021, Jul. 18, 1995] applied to a metal anode a protective surface layer (e.g., a mixture of polynuclear aromatic and polyethylene oxide) that enables transfer of metal ions from the metal anode to the electrolyte and back. The surface layer is also electronically conductive so that the ions will be uniformly attracted back onto the metal anode during electrodeposition (i.e. during battery recharge). Alamgir, et al. [M. Alamgir, et al. “Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat. No. 5,536,599, Jul. 16, 1996] used ferrocenes to prevent chemical overcharge and dendrite formation in a solid polymer electrolyte-based rechargeable battery.

Skotheim [T. A. Skotheim, “Stabilized Anode for Lithium-Polymer Battery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No. 5,961,672 (Oct. 5, 1999)] provided a Li metal anode that was stabilized against the dendrite formation by the use of a vacuum-evaporated thin film of a Li ion-conducting polymer interposed between the Li metal anode and the electrolyte. Skotheim, et al. [T. A. Skotheim, et al. “Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May 11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No. 6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007)] further proposed a multilayer anode structure consisting of a Li metal-based first layer, a second layer of a temporary protective metal (e.g., Cu, Mg, and Al), and a third layer that is composed of at least one layer (typically 2 or more layers) of a single ion-conducting glass, such as lithium silicate and lithium phosphate, or polymer. It is clear that such an anode structure, consisting of at least 3 or 4 layers, is too complex and too costly to make and use.

Protective coatings for Li anodes, such as glassy surface layers of LiI— Li₃PO₄—P₂S₅, may be obtained from plasma assisted deposition [S. J. Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat. No. 6,025,094 (Feb. 15, 2000)]. Complex, multi-layer protective coatings were also proposed by Visco, et al. [S. J. Visco, et al., “Protected Active Metal Electrode and Battery Cell Structures with Non-aqueous Interlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S. Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct. 16, 2007)].

Despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. Solid electrolytes typically have a low lithium ion conductivity, are difficult to produce and difficult to implement into a battery.

Furthermore, solid electrolyte, as the sole electrolyte in a cell or as an anode-protecting layer (interposed between the lithium film and another electrolyte) does not have and cannot maintain a good contact with the lithium metal. This effectively reduces the effectiveness of the electrolyte to support dissolution of lithium ions (during battery discharge), transport lithium ions, and allowing the lithium ions to re-deposit back to the lithium anode (during battery recharge). A ceramic separator that is disposed between an anode active material layer (e.g. a graphite-based anode layer or a lithium metal layer) and a cathode active layer suffers from the same problems as well. In addition, a ceramic separator also has a poor contact with the cathode layer if the electrolyte in the cathode layer is a solid electrolyte (e.g. inorganic solid electrolyte).

Another major issue associated with the lithium metal anode is the continuing reactions between electrolyte and lithium metal, leading to repeated formation of “dead lithium-containing species” that cannot be re-deposited back to the anode and become isolated from the anode. These reactions continue to irreversibly consume electrolyte and lithium metal, resulting in rapid capacity decay. In order to compensate for this continuing loss of lithium metal, an excessive amount of lithium metal (3-5 times higher amount than what would be required) is typically implemented at the anode when the battery is made. This adds not only costs but also a significant weight and volume to a battery, reducing the energy density of the battery cell. This important issue has been largely ignored and there has been no plausible solution to this problem in battery industry.

Clearly, an urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries, and to reducing or eliminating the detrimental reactions between lithium metal and the electrolyte.

Hence, an object of the present disclosure was to provide a multi-functional separator to overcome the lithium metal dendrite formation and penetration problems and to prevent continuous electrolyte-lithium reactions in all types of Li metal batteries having a lithium metal anode. A specific object of the present disclosure was to provide a lithium metal cell or a lithium-ion cell that exhibits a safe, high specific capacity, high specific energy, high degree of safety, and a long and stable cycle life.

SUMMARY

The present disclosure provides a lithium secondary battery comprising a cathode, an anode, an elastic and flame retardant composite separator (ion-conducting membrane) disposed between the cathode and the anode, and a working electrolyte through which lithium ions are transported between the anode and the cathode during the battery charging or discharging step, wherein the elastic and flame retardant composite separator comprises a high-elasticity polymer and from 0.1% to 95% (preferably greater than 10%, more preferably from 20% to 85%, and most preferably from 30% to 70%) by weight of a flame retardant that is dispersed in, dissolved in, or chemically bonded to the high-elasticity polymer, wherein the elastic flame retardant composite separator has a thickness from 50 nm to 100 μm (preferably from 1 to 20 μm), and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm at room temperature and the high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% (preferably greater than 10% and further preferably from 30% to 300%) when measured without any additive dispersed therein. In a typical configuration, the separator is in ionic contact with both the anode and the cathode and typically in physical contact with an anode active material layer (or an anode current collector) and with a cathode active material layer.

With this multi-functional separator, there is no need to have a separate or additional separator or anode-protecting layer in the battery cell. This multi-functional separator serves not only as a separator that electrically isolates the anode from the cathode but also a lithium metal protection layer (in the cases where lithium metal is the primary anode active material). This separator layer is elastic, enabling good contact between this separator layer and both the anode active layer (or anode current collector) and the cathode active layer, thereby significantly reducing the interfacial impedance. This layer also acts to provide flame-retardant or fire-resisting capability to the battery.

In certain embodiments, the anode in the lithium secondary battery has an amount of lithium or lithium alloy as an anode active material supported by an anode current collector. In certain other embodiments, initially the anode has no lithium or lithium alloy as an anode active material supported by said anode current collector when the battery is made and prior to a charge or discharge operation of the battery. The needed lithium ions are pre-stored in the cathode active material when the battery is made. This latter configuration is referred to as an anode-less lithium battery.

In certain embodiments, the battery is a lithium-ion battery and the anode has an anode current collector and a layer of an anode active material supported by the anode current collector, which is in physical contact with the elastic flame-retardant composite separator. The anode active materials is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), phosphorus (P), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium titanium niobium oxide, lithium-containing titanium oxide, lithium transition metal oxide, ZnCo₂O₄; (f) carbon or graphite particles (g) prelithiated versions thereof; and (h) combinations thereof.

The anode current collector may be selected from, for instance, a Cu foil, a Cu-coated polymer film, a sheet of Ni foam, a porous layer of nano-filaments, such as graphene sheets, carbon nanofibers, carbon nano-tubes, etc.

Preferably, the flame retardant additive is selected from a halogenated flame retardant, phosphorus-based flame retardant, melamine flame retardant, metal hydroxide flame retardant, silicon-based flame retardant, phosphate flame retardant, biomolecular flame retardant, or a combination thereof.

In certain embodiments, the elastic polymer composite comprises reactive type flame retardant (flame-retardant group becomes part of the polymer chain structure after polymerization or crosslinking), additive type flame retardant (additive simply dispersed in the polymer matrix), or both types. For instance, the elastic polymer composite may comprise a flame retardant chemical group that is bonded to polysiloxane, which is elastic.

In certain embodiments, the flame retardant additive is in a form of encapsulated particles comprising the additive encapsulated by a shell of coating material that is breakable or meltable when exposed to a temperature higher than a threshold temperature (e.g., flame or fire temperature induced by internal shorting). The encapsulating material is a substantially lithium ion-impermeable and liquid electrolyte-impermeable coating material.

In the lithium secondary battery, the working electrolyte is selected from an organic liquid electrolyte, ionic liquid electrolyte, polymer gel electrolyte, solid-state electrolyte, quasi-solid electrolyte having a lithium salt dissolved in an organic or ionic liquid with a lithium salt concentration higher than 2.0 M, hybrid or composite electrolyte, or a combination thereof.

Preferably, the elastic flame-retardant composite separator layer further comprises from 1% to 90% by weight of particles of an inorganic solid electrolyte material dispersed therein wherein the particles have a particle size preferably from 10 nm to 30 μm, more preferably from 50 nm to 1 μm. The inorganic solid electrolyte material may be selected from an oxide type, sulfide type, hydride type, halide type, borate type, phosphate type, lithium phosphorus oxynitride (LiPON), Garnet-type, lithium superionic conductor (LISICON), sodium superionic conductor (NASICON), or a combination thereof.

A high-elasticity polymer refers to a polymer, typically a lightly cross-linked polymer, which exhibits an elastic deformation that is at least 2% (preferably at least 5%) when measured under uniaxial tension. In the field of materials science and engineering, the “elastic deformation” is defined as a deformation of a material (when being mechanically stressed) that is essentially fully recoverable upon release of the load and the recovery process is essentially instantaneous (no or little time delay). The elastic deformation is more preferably greater than 10%, even more preferably greater than 30%, further more preferably greater than 50%, and still more preferably greater than 100%. The elasticity of the elastic polymer alone (without any additive dispersed therein) can be as high as 1,000%. However, the elasticity can be significantly reduced if a certain amount of inorganic filler is added into the polymer. Depending upon the type and proportion of the solid electrolyte particles incorporated, the reversible elastic deformation is typically reduced to the range of 2%-500%, more typically 2%-300%.

In some preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network of polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof, in the cross-linked network of polymer chains. These network or cross-linked polymers exhibit a unique combination of a high elasticity (high elastic deformation strain) and high lithium-ion conductivity.

In certain preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate (PETEA) chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, acrylic acid-derived chains, polyvinyl alcohol chains, or a combination thereof.

The elastic polymer may comprise an elastomer selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polysiloxane, polyurethane, urethane-urea copolymer, urethane-acrylic copolymer, a copolymer thereof, a sulfonated version thereof, or a combination thereof.

In some embodiments, the high-elasticity polymer comprises an elastomer selected from natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, and combinations thereof.

The high elasticity polymer may further comprise from 0.1% to 50% by weight of a lithium ion-conducting additive, which is different from the inorganic solid electrolyte particles.

In certain embodiments, the anode contains a current collector without a lithium metal or any other anode active material, such as graphite or Si particles, when the battery cell is manufactured. Such a battery cell having an initially lithium metal-free anode is commonly referred to as an “anode-less” lithium battery. The lithium ions that are required for shuttling back and forth between the anode and the cathode are initially stored in the cathode active materials (e.g. Li in LiMn₂O₄ and LiMPO₄, where M=Ni, Co, F, Mn, etc.). During the first battery charge procedure, lithium ions (Li⁺) come out of the cathode active material, move through the electrolyte and then through the presently disclosed elastic and flame retardant composite separator and get deposited on a surface of the anode current collector. As this charging procedure continues, more lithium ions get deposited onto the current collector surface, eventually forming a lithium metal film or coating. The high-elasticity nature of the disclosed separator may be squeezed when the lithium film increases in thickness.

During the subsequent discharge, this lithium film or coating layer decreases in thickness due to dissolution of lithium into the electrolyte to become lithium ions, possibly creating a gap between the current collector and the protective layer if the separator layer were not elastic. Such a gap would make the re-deposition of lithium ions back to the anode impossible during a subsequent recharge procedure. We have observed that the elastic composite separator is capable of expanding or shrinking congruently or conformably with the anode layer. This capability helps to maintain a good contact between the current collector (or the lithium film itself) and the protective layer, enabling the re-deposition of lithium ions without interruption.

In certain embodiments, the high-elasticity polymer further contains a reinforcement material dispersed therein wherein the reinforcement material is selected from a polymer fiber, a glass fiber, a ceramic fiber or nano-flake (e.g. nano clay flakes), or a combination thereof. The reinforcement material preferably has a thickness or diameter less than 100 nm.

The elastic composite separator may further comprise a lithium salt (as a lithium ion-conducting additive) dispersed in the polymer wherein the lithium salt may be preferably selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

The composite separator preferably has a lithium ion conductivity no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴ S/cm, and most preferably no less than 10⁻³ S/cm. Some of the selected polymers exhibit a lithium-ion conductivity greater than 10⁻² S/cm.

In some embodiments, the high-elasticity polymer composite further comprises a lithium ion-conducting additive dispersed in a high-elasticity polymer matrix material, wherein the lithium ion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

In some embodiments, the high-elasticity polymer forms a mixture, blend, or semi-IPN with a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof. Sulfonation is herein found to impart improved lithium ion conductivity to a polymer.

The cathode active material may be selected from an inorganic material, an organic material, a polymeric material, or a combination thereof. The inorganic material may be selected from sulfur, selenium, a metal oxide, metal phosphate, metal silicide, metal selenide (e.g. lithium polyselides for use in a Li—Se cell), metal sulfide (e.g. lithium polysulfide for use in a Li—S cell), or a combination thereof. Preferably, these cathode active materials contain lithium in their structures; otherwise the cathode should contain a lithium source.

The inorganic cathode active material may be selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, lithium metal silicide, or a combination thereof.

The cathode active material layer may contain a metal oxide containing vanadium oxide selected from the group consisting of Li_(x)VO₂, Li_(x)V₂O₅, Li_(x)V₃O₈, Li_(x)V₃O₇, Li_(x)V₄O₉, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metal phosphate, selected from a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

The cathode active material is preferably in a form of nano particle (spherical, ellipsoidal, and irregular shape), nano wire, nano fiber, nano tube, nano sheet, nano belt, nano ribbon, nano disc, nano platelet, or nano horn having a thickness or diameter less than 100 nm. These shapes can be collectively referred to as “particles” unless otherwise specified or unless a specific type among the above species is desired. Further preferably, the cathode active material has a dimension less than 50 nm, even more preferably less than 20 nm, and most preferably less than 10 nm. In some embodiments, one particle or a cluster of particles may be coated with or embraced by a layer of carbon disposed between the particle(s) and/or a high-elasticity polymer layer (an encapsulating shell).

The cathode layer may further contain a graphite, graphene, or carbon material mixed with the cathode active material particles. The carbon or graphite material is selected from polymeric carbon, amorphous carbon, chemical vapor deposition carbon, coal tar pitch, petroleum pitch, meso-phase pitch, carbon black, coke, acetylene black, activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, natural graphite particle, or a combination thereof. Graphene may be selected from pristine graphene, graphene oxide, reduced graphene oxide, graphene fluoride, hydrogenated graphene, nitrogenated graphene, functionalized graphene, etc.

The cathode active material particles may be coated with or embraced by a conductive protective coating, selected from a carbon material, graphene, electronically conductive polymer, conductive metal oxide, or conductive metal coating.

In certain embodiments, the elastic composite layer has two primary surfaces with a first primary surface facing the anode side and a second primary surface opposing or opposite to the first primary surface (facing the cathode side) and wherein the flame-retardant and/or optional solid electrolyte powder has a first concentration at the first surface and a second concentration at the second surface and the first concentration is greater than the second concentration. In other words, there are more flame retardant and/or inorganic particles at anode side of the elastic composite separator layer than the opposite side intended to be facing the cathode. There is a concentration gradient across the thickness of the elastic composite separator layer. The high concentration of the flame retardant and/or inorganic solid electrolyte particles on the anode side (preferably >30% by weight and more preferably >60% by weight) can help stop the penetration of any lithium dendrite, if formed, and help to form a stable artificial solid-electrolyte interphase (SEI). The high concentration of a flame retardant facing the anode side also acts to suppress any internal thermal run-away or fire. Thus, in some embodiments, the elastic composite separator has a gradient concentration of the flame retardant and/or the inorganic solid electrolyte particles across the thickness of the separator.

The disclosure also provides a process for manufacturing the elastic composite separator, the process comprising (A) dispersing a flame retardant additive and, optionally particles of the inorganic solid electrolyte particles, in a liquid reactive mass of an elastic polymer precursor to form a slurry; (B) dispensing and depositing a layer of the liquid reactive mass onto a solid substrate surface; and (C) polymerizing and/or curing (crosslinking) the reactive mass to form a layer of elastic composite separator.

The solid substrate may be an anode current collector, an anode active material layer, or a cathode active material layer. In other words, this elastic composite separator can be directly deposited onto a layer of anode active material, an anode current collector, or a layer of cathode active material. This is achievable because curing of the high-elasticity polymer does not require a high temperature; curing temperature typically lower than 200° C. or even lower than 100° C. This is in stark contrast to the typically 900-1,200° C. required of sintering an inorganic solid electrolyte to form a ceramic separator. In addition, the presently disclosed elastic composite separator is at least as good as a ceramic separator in terms of reducing interfacial impedance and stopping dendrite penetration.

Preferably, the process is a roll-to-roll process wherein step (B) comprises (i) continuously feeding a layer of the solid substrate (e.g. flexible metal film, plastic film, etc.) from a feeder roller to a dispensing zone where the reactive mass is dispensed and deposited onto the solid substrate to form a continuous layer of the reactive mass; (ii) moving the layer of the reactive mass into a reacting zone where the reactive mass is exposed to heat, ultraviolet (UV) light, or high-energy radiation to polymerize and/or crosslink the reactive mass to form a continuous layer or roll of elastic composite; and (iii) collecting the elastic composite on a winding roller. This process is conducted in a reel-to-reel manner.

The process may further comprise unwinding the elastic composite roll or layer from the winding roller and cutting/trimming the roll (or part of the roll) of elastic composite into one or multiple pieces of elastic composite layers.

The process may further comprise combining an anode, the elastic composite separator, a working electrolyte, and a cathode electrode to form a lithium battery.

The disclosure also provides an elastic and flame retardant composite layer, wherein the elastic and flame retardant composite comprises a high-elasticity polymer and from 0.1% to 95% by weight of a flame retardant additive dispersed in, dissolved in, or chemically bonded to the high-elasticity polymer, wherein said elastic composite separator has a thickness from 50 nm to 100 μm and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm at room temperature and said high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% when measured without any additive dispersed therein.

Preferably, the flame retardant additive is in a form of encapsulated particles comprising the additive encapsulated by a shell of coating material that is breakable or meltable when exposed to a temperature higher than a threshold temperature (e.g., flame or fire temperature induced by internal shorting). The encapsulating material is a substantially lithium ion-impermeable and liquid electrolyte-impermeable coating material.

Such an elastic, ion-conducting, and flame-retardant composite may be used as a separator, an anode protection layer, or both for a lithium secondary battery.

Preferably, the high-elasticity polymer composite has a lithium ion conductivity from 1×10⁻⁵ S/cm to 5×10⁻² S/cm. In some embodiments, the high-elasticity polymer composite has a recoverable tensile strain from 10% to 300% (more preferably >30%, and further more preferably >50%).

In certain embodiments, some additive, such as particles of a solid inorganic electrolyte, an elastomer (or its precursor), an ion-conductive polymer, a lithium-ion conducting material, a reinforcement material (e.g. high-strength, non-conducting fibers), or a combination thereof may be added into the reactive mass.

The lithium ion-conducting material is dispersed in the high-elasticity polymer and is preferably selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

In some embodiments, the lithium ion-conducting material dispersed in the reactive mass is selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

If desired, the resulting elastic composite separator may be soaked in or impregnated with an organic or ionic liquid electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a prior art lithium metal battery cell, containing an anode layer (a thin Li foil or Li coating deposited on a surface of a current collector, Cu foil), a porous separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown.

FIG. 2 Schematic of a presently disclosed lithium metal battery cell (upper diagram) containing an anode current collector (e.g. Cu foil) but no anode active material (when the cell is manufactured or in a fully discharged state), an elastic composite separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector supporting the cathode active layer is also shown. The lower diagram shows a thin lithium metal layer deposited between the Cu foil and the elastic composite separator layer when the battery is in a charged state.

FIG. 3(A) Schematic of an elastic composite separator layer wherein the flame retardant and/or inorganic solid electrolyte particles are uniformly dispersed in a matrix of high-elasticity polymer according to some embodiments of the present disclosure;

FIG. 3(B) Schematic of an elastic composite separator layer wherein the flame retardant and/or inorganic solid electrolyte particles are preferentially dispersed near one surface (e.g. facing the anode side) of an elastic composite separator layer; the opposing surface has a lower or zero concentration of the flame retardant and/or inorganic solid electrolyte particles, according to some embodiments of the present disclosure.

FIG. 4 Schematic of a roll-to-roll process for producing rolls of elastic composite separator in a continuous manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure is related to a lithium secondary battery, which is preferably based on an organic electrolyte, a polymer gel electrolyte, an ionic liquid electrolyte, a quasi-solid electrolyte, or a solid-state electrolyte. The shape of a lithium secondary battery can be cylindrical, square, button-like, etc. The present disclosure is not limited to any battery shape or configuration or any type of electrolyte.

The present disclosure provides a lithium secondary battery comprising a cathode, an anode, an elastic and flame-resistant composite separator (ion-conducting membrane) disposed between the cathode and the anode, and a working electrolyte. The elastic flame-retardant composite separator comprises a high-elasticity polymer and from 1% to 99% (preferably greater than 10%, more preferably from 20% to 95%, and most preferably from 30% to 90%) by weight of a flame retardant additive, optionally along with particles of an inorganic solid electrolyte. The flame retardant additive is dissolved in, dispersed in, or chemically bonded by the high-elasticity polymer. The elastic composite separator has a thickness from 50 nm to 100 μm (preferably from 1 to 20 μm), and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm at room temperature and the high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% (preferably greater than 10% and further preferably from 30% to 300%) when measured without any additive dispersed therein. In a typical configuration, the separator is in ionic contact with both the anode and the cathode and typically in physical contact with an anode active material layer (or an anode current collector) and with a cathode active material layer.

In certain embodiments, the anode in the lithium secondary battery has an amount of lithium or lithium alloy as an anode active material supported by an anode current collector. In certain other embodiments, initially the anode has no lithium or lithium alloy as an anode active material supported by an anode current collector when the battery is made and prior to a charge or discharge operation of the battery. This latter configuration is referred to as an anode-less lithium battery. In certain embodiments the battery can be manufactured with a lithium metal film between the elastic, flame retardant composite separator and the anode current collector.

In certain embodiments, the battery is a lithium-ion battery and the anode has an anode current collector and a layer of an anode active material supported by the anode current collector, which is in physical contact with the elastic flame-retardant composite separator. The anode active materials is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), phosphorus (P), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium titanium niobium oxide, lithium-containing titanium oxide, lithium transition metal oxide, ZnCo₂O₄; (f) carbon or graphite particles (g) prelithiated versions thereof; and (h) combinations thereof.

The current collector may be a Cu foil, a layer of Ni foam, a porous layer of nano-filaments, such as graphene sheets, carbon nanofibers, carbon nano-tubes, etc. forming a 3D interconnected network of electron-conducting pathways.

Preferably and most typically, this elastic composite separator layer is different in composition than the working electrolyte used in the lithium battery and the elastic composite layer maintains as a discrete layer (not to be dissolved in the electrolyte).

We have discovered that this composite ceramic layer provides several unexpected benefits: (a) the formation and penetration of dendrite can be essentially eliminated; (b) uniform deposition of lithium back to the anode side is readily achieved during battery charging; (c) the layer ensures smooth and uninterrupted transport of lithium ions from/to the anode current collector surface (or the lithium film deposited thereon during the battery operations) and through the interface between the current collector (or the lithium film deposited thereon) and the elastic composite separator layer with minimal interfacial resistance; (d) flame and fire-fighting capability is intrinsically built into the battery; (e) reduced/eliminated electrolyte/lithium metal reactions; (e) reduced interfacial resistance at both the anode side and the cathode side; and (f) cycle stability can be significantly improved and cycle life increased. No additional protective layer for the lithium metal anode is required. The separator itself also plays the role as an anode protective layer.

In a conventional lithium metal cell, as illustrated in FIG. 1, the anode active material (lithium) is deposited in a thin film form or a thin foil form directly onto an anode current collector (e.g. a Cu foil) before this anode and a cathode are combined to form a cell. The battery is a lithium metal battery, lithium sulfur battery, lithium-selenium battery, etc. As previously discussed in the Background section, these lithium secondary batteries have the dendrite-induced internal shorting and “dead lithium” issues at the anode.

We have solved these challenging issues that have troubled battery designers and electrochemists alike for more than 30 years by developing and implementing a new elastic composite separator disposed between the anode (an anode current collector or an anode active material layer) and a cathode active material layer. This elastic separator layer comprises a high-elasticity polymer composite having a recoverable (elastic) tensile strain no less than 2% (preferably no less than 5%, and further preferably from 10% to 500%) under uniaxial tension and a lithium ion conductivity no less than 10⁻⁸ S/cm at room temperature (preferably and more typically from 1×10⁻⁵ S/cm to 5×10⁻² S/cm).

As schematically shown in FIG. 2, one embodiment of the present disclosure is a lithium metal battery or lithium-ion cell containing an anode current collector (e.g. Cu foil), a high-elasticity flame-retardant composite-based composite separator, and a cathode active material layer, which is composed of particles of a cathode active material, a conductive additive (not shown) and a resin binder (not shown). A cathode current collector (e.g. Al foil) supporting the cathode active layer is also shown in FIG. 2.

The high-elasticity polymer material refers to a material (polymer or polymer composite) that exhibits an elastic deformation of at least 2% when measured under uniaxial tension. In the field of materials science and engineering, the “elastic deformation” is defined as a deformation of a material (when being mechanically stressed) that is essentially fully recoverable and the recovery is essentially instantaneous upon release of the load. The elastic deformation is preferably greater than 5%, more preferably greater than 10%, further more preferably greater than 30%, and still more preferably greater than 100% (up to 500%).

It may be noted that FIG. 2 shows a lithium battery that initially does not contain a lithium foil or lithium coating at the anode (only an anode current collector, such as a Cu foil or a graphene/CNT mat) when the battery is made. The needed lithium to be bounced back and forth between the anode and the cathode is initially stored in the cathode active material (e.g. lithium vanadium oxide Li_(x)V₂O₅, instead of vanadium oxide, V₂O₅; or lithium polysulfide, instead of sulfur). During the first charging procedure of the lithium battery (e.g. as part of the electrochemical formation process), lithium comes out of the cathode active material, passes through the elastic composite separator and deposits on the anode current collector. The presence of the presently disclosed high-elasticity composite separator (in good contact with the current collector) enables the uniform deposition of lithium ions on the anode current collector surface. Such a battery configuration avoids the need to have a layer of lithium foil or coating being present during battery fabrication. Bare lithium metal is highly sensitive to air moisture and oxygen and, thus, is more challenging to handle in a real battery manufacturing environment. This strategy of pre-storing lithium in the lithiated (lithium-containing) cathode active materials, such as Li_(x)V₂O₅ and Li₂S_(x), makes all the materials safe to handle in a real manufacturing environment. Cathode active materials, such as Li_(x)V₂O₅ and Li₂S_(x), are typically not air-sensitive.

As the charging procedure continues, more lithium ions get to deposit onto the anode current collector, forming a lithium metal film or coating. During the subsequent discharge procedure, this lithium film or coating layer decreases in thickness due to dissolution of lithium into the electrolyte to become lithium ions, possibly creating a gap between the current collector and the separator layer if the separator layer were not elastic (e.g. a ceramic separator). Such a gap would make the re-deposition of lithium ions back to the anode impossible during a subsequent recharge procedure. We have observed that the presently disclosed elastic composite separator layer is capable of expanding or shrinking congruently or conformably with the anode layer. This capability helps to maintain a good contact between the current collector (or the lithium film subsequently or initially deposited on the current collector surface) and the protective layer, enabling the re-deposition of lithium ions without interruption.

FIG. 3(A) schematically shows an elastic composite separator layer wherein the flame retardant additive and the optional inorganic solid electrolyte particles are uniformly dispersed in a matrix of an elastic polymer according to some embodiments of the present disclosure. According to some other embodiments of the present disclosure, FIG. 3(B) schematically shows an elastic composite separator layer wherein the flame retardant additive and inorganic solid electrolyte particles are preferentially dispersed near one surface (e.g. facing the anode side) of an elastic composite separator layer; the opposing surface has a lower or zero concentration of the inorganic solid electrolyte particles. This latter structure has the advantages that the high-concentration portion, being strong and rigid, provides a lithium dendrite-stopping capability while other portion of the layer remains highly elastic to maintain good contacts with neighboring layers (e.g. cathode active material layer containing a solid electrolyte on one side and lithium metal on the other) for reduced interfacial impedance. The elastic flame retardant composite separator also acts to retard the flame or fight any internal thermal runaway issue.

Flame-retardant additives are intended to inhibit or stop polymer pyrolysis and combustion processes by interfering with the various mechanisms involved—heating, ignition, and propagation of thermal degradation.

The flame retardant additive may be selected from a halogenated flame retardant, phosphorus-based flame retardant, melamine flame retardant, metal hydroxide flame retardant, silicon-based flame retardant, phosphate flame retardant, biomolecular flame retardant, or a combination thereof.

There is no limitation on the type of flame retardant that can be physically or chemically incorporated into the elastic polymer. The main families of flame retardants are based on compounds containing: Halogens (Bromine and Chlorine), Phosphorus, Nitrogen, Intumescent Systems, Minerals (based on aluminum and magnesium), and others (e.g. Borax, Sb₂O₃, and nanocomposites). Antimony trioxide is a good choice, but other forms of antimony such as the pentoxide and sodium antimonate may also be used.

One may use the reactive types (being chemically bonded to or becoming part of the polymer structure) and additive types (simply dispersed in the polymer matrix). For instance, reactive polysiloxane can chemically react with EPDM type elastic polymer and become part of the crosslinked network polymer. It may be noted that flame-retarding group modified polysiloxane itself is an elastic polymer composite containing a flame reatardant according to an embodiment of instant disclosure. Both reactive and additive types of flame retardants can be further separated into several different classes:

-   -   1) Minerals: Examples include aluminum hydroxide (ATH),         magnesium hydroxide (MDH), huntite and hydromagnesite, various         hydrates, red phosphorus and boron compounds (e.g. borates).     -   2) Organo-halogen compounds: This class includes organochlorines         such as chlorendic acid derivatives and chlorinated paraffins;         organobromines such as decabromodiphenyl ether (decaBDE),         decabromodiphenyl ethane (a replacement for decaBDE), polymeric         brominated compounds such as brominated polystyrenes, brominated         carbonate oligomers (BCOs), brominated epoxy oligomers (BEOs),         tetrabromophthalic anyhydride, tetrabromobisphenol A (TBBPA),         and hexabromocyclododecane (HBCD).     -   3) Organophosphorus compounds: This class includes         organophosphates such as triphenyl phosphate (TPP), resorcinol         bis(diphenylphosphate) (RDP), bisphenol A diphenyl phosphate         (BADP), and tricresyl phosphate (TCP); phosphonates such as         dimethyl methylphosphonate (DMMP); and phosphinates such as         aluminum diethyl phosphinate. In one important class of flame         retardants, compounds contain both phosphorus and a halogen.         Such compounds include tris(2,3-dibromopropyl) phosphate         (brominated tris) and chlorinated organophosphates such as         tris(1,3-dichloro-2-propyl)phosphate (chlorinated tris or TDCPP)         and tetrakis(2-chlorethyl) dichloroisopentyldiphosphate (V6).     -   4) Organic compounds such as carboxylic acid and dicarboxylic         acid

The mineral flame retardants mainly act as additive flame retardants and do not become chemically attached to the surrounding system (the polymer). Most of the organohalogen and organophosphate compounds also do not react permanently to attach themselves into the polymer. Certain new non halogenated products, with reactive and non-emissive characteristics have been commercially available as well.

In certain embodiments, the flame retardant additive is in a form of encapsulated particles comprising the additive encapsulated by a shell of coating material that is breakable or meltable when exposed to a temperature higher than a threshold temperature (e.g., flame or fire temperature induced by internal shorting). The threshold temperature is not particularly limited, a threshold temperature typically ranges from 30° C. to 200° C. The threshold temperature can be any temperature at which the shell of coating material is made breakable or meltable. The encapsulating material is a substantially lithium ion-impermeable and liquid electrolyte-impermeable coating material. The encapsulating or micro-droplet formation processes that can be used to produce protected flame-retardant particles are briefly described below. The threshold temperature can be adjusted by modifying the encapsulating material.

Several composite droplet forming processes require the encapsulating polymer or its precursor (monomer or oligomer) to be dissolvable in a solvent. Fortunately, there are a wide variety of polymers or their precursors used herein are soluble in some common solvents or water; water being the preferred liquid solvent. The un-cured polymer or its precursor can be readily dissolved in a common organic solvent or water to form a solution. This solution can then be used to embed, immerse, engulf or encapsulate the solid particles (e.g. flame retardant aluminum hydroxide particles and magnesium hydroxide particles) via several of the micro-droplet-forming methods to be discussed in what follows. Upon formation of the droplets, the polymer matrix is then polymerized and cross-linked.

There are three broad categories of micro-encapsulation methods that can be implemented to produce polymer-encapsulated flame retardant: physical methods, physico-chemical methods, and chemical methods. The physical methods include extrusion and pelletizing, solution dipping and drying, suspension coating or casting on a solid substrate (e.g. slot-die coating, Comma coating, spray-coating) followed by drying and scratching off particles from the substrate, pan-coating, air-suspension coating, centrifugal extrusion, vibration nozzle, and spray-drying methods. The physico-chemical methods include ionotropic gelation and coacervation-phase separation methods. The chemical methods include interfacial polycondensation, interfacial cross-linking, in-situ polymerization, and matrix polymerization.

It may be noted that some of these methods (e.g. pan-coating, air-suspension coating, and spray-drying) may be used to coat or encapsulate particles by adjusting the solid content, degree of dispersion, spraying and drying conditions, etc.

Pan-coating method: The pan coating process involves tumbling the flame retardant particles in a pan or a similar device while the matrix material (e.g. monomer/oligomer liquid or uncured polymer/solvent solution; possibly containing a lithium salt dispersed or dissolved therein) is applied slowly until a desired amount of matrix is attained.

Air-suspension coating method: In the air suspension coating process, the solid particles are dispersed into the supporting air stream in an embedding chamber. A controlled stream of a reactive precursor solution (e.g. polymer or its monomer or oligomer dissolved in a solvent; or its monomer or oligomer alone in a liquid state) is concurrently introduced into this chamber, allowing the solution to hit and embed the suspended particles. These suspended particles are embedded in the reactive precursor (monomer, oligomer, etc. which is polymerized/cured concurrently or subsequently) while the volatile solvent is removed, leaving behind a composite comprising a matrix of conducting network polymer and anode active material particles. This process may be repeated several times until the required parameters, such as full embedding, are achieved. The air stream which supports the anode particles also helps to dry them, and the rate of drying is directly proportional to the temperature of the air stream, which can be adjusted for optimized polymer network amount.

In a preferred mode, the particles in the embedding zone portion may be subjected to re-circulation for repeated embedding. Preferably, the chamber is arranged such that the particles pass upwards through the embedding zone, then are dispersed into slower moving air and sink back to the base of the encapsulating chamber, enabling repeated passes of the particles through the embedding zone until the desired matrix amount is achieved.

Centrifugal extrusion: Additive particles may be embedded in a polymer network or precursor material using a rotating extrusion head containing concentric nozzles. In this process, a stream of core fluid (slurry containing additive particles dispersed in a solvent) is surrounded by a sheath of shell solution or melt containing the polymer or precursor. As the device rotates and the stream moves through the air it breaks, due to Rayleigh instability, into droplets of core, each coated with the shell solution. While the droplets are in flight, the molten shell may be hardened or the solvent may be evaporated from the shell solution. If needed, the capsules can be hardened after formation by catching them in a hardening bath. Since the drops are formed by the breakup of a liquid stream, the process is only suitable for liquid or slurry. A high production rate can be achieved. Up to 22.5 kg of microcapsules can be produced per nozzle per hour and extrusion heads containing 16 nozzles are readily available.

Vibrational nozzle encapsulation method: matrix-encapsulation of particles can be conducted using a laminar flow through a nozzle and vibration of the nozzle or the liquid. The vibration has to be done in resonance with the Rayleigh instability, leading to very uniform droplets. The liquid can include any liquids with limited viscosities (1-50,000 mPa·s): emulsions, suspensions or slurry containing the anode active material particles and the polymer or precursor.

Spray-drying: Spray drying may be used to encapsulate and embed particles when the particles are suspended in a melt or polymer/precursor solution to form a suspension. In spray drying, the liquid feed (solution or suspension) is atomized to form droplets which, upon contacts with hot gas, allow solvent to get vaporized and a polymer matrix to fully embed the particles.

In-situ polymerization: In some micro-encapsulation processes, particles are fully embedded in a monomer or oligomer first. Then, direct polymerization of the monomer or oligomer is carried out with the presence of these material particles dispersed therein.

Matrix polymerization: This method involves dispersing and embedding solid particles in a polymeric matrix during formation of the particles. This can be accomplished via spray-drying, in which the particles are formed by evaporation of the solvent from the matrix material. Another possible route is the notion that the solidification of the matrix is caused by a chemical change.

Preferably, the inorganic solid electrolyte material (to be added into the elastic composite separator as a lithium ion conductivity enhancer and a lithium dendrite stopper) is in a fine powder form having a particle size preferably from 10 nm to 30 μm (more preferably from 50 nm to 1 μm). The inorganic solid electrolyte material may be selected from an oxide type (e.g. perovskite-type), sulfide type, hydride type, halide type, borate type, phosphate type, lithium phosphorus oxynitride (LiPON), Garnet-type, lithium superionic conductor (LISICON), sodium superionic conductor (NASICON), or a combination thereof.

The inorganic solid electrolytes that can be incorporated into an elastic separator include, but are not limited to, perovskite-type, NASICON-type, garnet-type and sulfide-type materials. A representative and well-known perovskite solid electrolyte is Li_(3x)La_(2/3−x)TiO₃, which exhibits a lithium-ion conductivity exceeding 10⁻³ S/cm at room temperature. This material has been deemed unsuitable in lithium batteries because of the reduction of Ti⁴⁺ on contact with lithium metal. However, we have found that this material, when dispersed in an elastic polymer, does not suffer from this problem.

The sodium superionic conductor (NASICON)-type compounds include a well-known Na_(1+x)Zr₂SidP_(3−x)O₁₂. These materials generally have an AM₂(PO₄)₃ formula with the A site occupied by Li, Na or K. The M site is usually occupied by Ge, Zr or Ti. In particular, the LiTi₂(PO₄)₃ system has been widely studied as a solid state electrolyte for the lithium-ion battery. The ionic conductivity of LiZr₂(PO₄)₃ is very low, but can be improved by the substitution of Hf or Sn. This can be further enhanced with substitution to form Li_(1+x)M_(x)Ti_(2−x)(PO₄)₃ (M=Al, Cr, Ga, Fe, Sc, In, Lu, Y or La). Al substitution has been demonstrated to be the most effective solid state electrolyte. The Li_(1+x)Al_(x)Ge_(2−x)(PO₄)₃ system s also an effective solid state due to its relatively wide electrochemical stability window. NASICON-type materials are considered as suitable solid electrolytes for high-voltage solid electrolyte batteries.

Garnet-type materials have the general formula A₃B₂Si₃O₁₂, in which the A and B cations have eightfold and sixfold coordination, respectively. In addition to Li₃M₂Ln₃O₁₂ (M=W or Te), a broad series of garnet-type materials may be used as an additive, including Li₅La₃M₂O₁₂ (M=Nb or Ta), Li₆ALa₂M₂O₁₂ (A=Ca, Sr or Ba; M=Nb or Ta), Li_(5.5)La₃M_(1.75)B_(0.25)O₁₂ (M=Nb or Ta; B=In or Zr) and the cubic systems Li₇La₃Zr₂O₁₂ and Li_(7.06)M₃Y_(0.06)Zr_(1.94)O₁₂ (M=La, Nb or Ta). The Li_(6.5)La₃Zr_(1.75)O₁₂ compounds have a high ionic conductivity of 1.02×10⁻³ S/cm at room temperature.

The sulfide-type solid electrolytes include the Li₂S—SiS₂ system. The highest repotted conductivity in this type of material is 6.9×10⁻⁴ S/cm, which was achieved by doping the Li₂S—SiS₂ system with Li₃PO₄. The sulfide type also includes a class of thio-LISICON (lithium superionic conductor) crystalline material represented by the Li₂S—P₂S₅ system. The chemical stability of the Li₂S—P₂S₅ system is considered as poor, and the material is sensitive to moisture (generating gaseous H₂S). The stability can be improved by the addition of metal oxides. The stability is also significantly improved if the Li₂S—P₂S₅ material is dispersed in an elastic polymer.

These solid electrolyte particles dispersed in an elastic polymer can help stop the penetration of lithium dendrites (if present) and enhance the lithium ion conductivity of certain elastic polymers having an intrinsically low ion conductivity.

Preferably and typically, the high-elasticity polymer has a lithium ion conductivity no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴ S/cm, further preferably no less than 10⁻³ S/cm, and most preferably no less than 10⁻² S/cm. The high-elasticity composite separator is a polymer matrix composite containing from 1% to 95% (preferably 10% to 85%) by weight of lithium ion-conducting solid electrolyte particles dispersed in or bonded by a high-elasticity polymer matrix material.

The high-elasticity polymer should have a high elasticity (elastic deformation strain value >2%). An elastic deformation is a deformation that is fully recoverable and the recovery process is essentially instantaneous (no significant time delay). The high-elasticity polymer can exhibit an elastic deformation from 5% up to 1,000% (10 times of its original length), more typically from 10% to 800%, and further more typically from 50% to 500%, and most typically and desirably from 70% to 300%. It may be noted that although a metal typically has a high ductility (i.e. can be extended to a large extent without breakage), the majority of the deformation is plastic deformation (non-recoverable) and only a small amount of elastic deformation (typically <1% and more typically <0.2%).

In some preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, acrylic acid-derived chains, polyvinyl alcohol chains, or a combination thereof, in the cross-linked network of polymer chains. These network or cross-linked polymers exhibit a unique combination of a high elasticity (high elastic deformation strain) and high lithium-ion conductivity.

In certain preferred embodiments, the high-elasticity polymer contains a lightly cross-linked network polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate (PETEA) chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, or a combination thereof.

Typically, a high-elasticity polymer is originally in a monomer or oligomer states that can be cured to form a cross-linked polymer that is highly elastic. Prior to curing, these polymers or oligomers are soluble in an organic solvent to form a polymer solution. An ion-conducting or flame retardant additive may be added to this solution to form a suspension. This solution or suspension can then be formed into a thin layer of polymer precursor on a surface of an anode current collector. The polymer precursor (monomer or oligomer and initiator) is then polymerized and cured to form a lightly cross-linked polymer. This thin layer of polymer may be tentatively deposited on a solid substrate (e.g. surface of a polymer or glass), dried, and separated from the substrate to become a free-standing polymer layer. This free-standing layer is then laid on a lithium foil/coating or implemented between a lithium film/coating and electrolyte or separator. Polymer layer formation can be accomplished by using one of several procedures well-known in the art; e.g. spraying, spray-painting, printing, coating, extrusion-based film-forming, casting, etc.

For instance, ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, chemical formula given below), along with an initiator, can be dissolved in an organic solvent, such as ethylene carbonate (EC) or diethyl carbonate (DEC). Then, the ETPTA monomer/solvent/initiator solution may be cast to form ETPTA a monomer/initiator layer on a glass surface. The layer can then be thermally cured to obtain a thin layer of a high-elasticity polymer. The polymerization and cross-linking reactions of this monomer can be initiated by a radical initiator derived from benzoyl peroxide (BPO) or AIBN through thermal decomposition of the initiator molecule. The ETPTA monomer has the following chemical formula:

As another example, the high-elasticity polymer for anode lithium foil/coating protection may be based on cationic polymerization and cross-linking of the cyanoethyl polyvinyl alcohol (PVA-CN, Formula 2) in succinonitrile (SN).

The procedure may begin with dissolving PVA-CN in succinonitrile (NCCH₂CH₂CN) to form a mixture solution. This is followed by adding an initiator into the mixture solution. For instance, LiPF₆ can be added into the PVA-CN/SN mixture solution at a weight ratio (selected from the preferred range from 20:1 to 2:1) to form a precursor solution. Then, the solution may be deposited to form a thin layer of reacting mass, PVA-CN/LiPF₆, which is subsequently heated at a temperature (e.g. from 75 to 100° C.) for 2 to 8 hours to obtain a high-elasticity polymer. During this process, cationic polymerization and cross-linking of cyano groups on the PVA-CN may be initiated by PF₅, which is derived from the thermal decomposition of LiPF₆ at such an elevated temperature.

It is essential for these materials to form a lightly cross-linked network of polymer chains. In other words, the network polymer or cross-linked polymer should have a relatively low degree of cross-linking or low cross-link density to impart a high elastic deformation.

The cross-link density of a cross-linked network of polymer chains may be defined as the inverse of the molecular weight between cross-links (Mc). The cross-link density can be determined by the equation, Mc=ρRT/Ge, where Ge is the equilibrium modulus as determined by a temperature sweep in dynamic mechanical analysis, p is the physical density, R is the universal gas constant in J/mol*K and T is absolute temperature in K. Once Ge and ρ are determined experimentally, then Mc and the cross-link density can be calculated.

The magnitude of Mc may be normalized by dividing the Mc value by the molecular weight of the characteristic repeat unit in the cross-link chain or chain linkage to obtain a number, Nc, which is the number of repeating units between two cross-link points. We have found that the elastic deformation strain correlates very well with Mc and Nc. The elasticity of a cross-linked polymer derives from a large number of repeating units (large Nc) between cross-links. The repeating units can assume a more relax conformation (e.g. random coil) when the polymer is not stressed. However, when the polymer is mechanically stressed, the linkage chain uncoils or gets stretched to provide a large deformation. A long chain linkage between cross-link points (larger Nc) enables a larger elastic deformation. Upon release of the load, the linkage chain returns to the more relaxed or coiled state. During mechanical loading of a polymer, the cross-links prevent slippage of chains that otherwise form plastic deformation (non-recoverable).

Preferably, the Nc value in a high-elasticity polymer is greater than 5, more preferably greater than 10, further more preferably greater than 100, and even more preferably greater than 200. These Nc values can be readily controlled and varied to achieve different elastic deformation values by using different cross-linking agents with different functionalities, and by designing the polymerization and cross-linking reactions to proceed at different temperatures for different periods of time.

Alternatively, Mooney-Rilvin method may be used to determine the degree of cross-linking. Crosslinking also can be measured by swelling experiments. In a swelling experiment, the crosslinked sample is placed into a good solvent for the corresponding linear polymer at a specific temperature, and either the change in mass or the change in volume is measured. The higher the degree of crosslinking, the less swelling is attainable. Based on the degree of swelling, the Flory Interaction Parameter (which relates the solvent interaction with the sample, Flory Huggins Eq.), and the density of the solvent, the theoretical degree of crosslinking can be calculated according to Flory's Network Theory. The Flory-Rehner Equation can be useful in the determination of cross-linking.

The high-elasticity polymer may contain a simultaneous interpenetrating network (SIN) polymer, wherein two cross-linking chains intertwine with each other, or a semi-interpenetrating network polymer (semi-IPN), which contains a cross-linked polymer and a linear polymer. An example of semi-IPN is an UV-curable/polymerizable trivalent/monovalent acrylate mixture, which is composed of ethoxylated trimethylolpropane triacrylate (ETPTA) and ethylene glycol methyl ether acrylate (EGMEA) oligomers. The ETPTA, bearing trivalent vinyl groups, is a photo (UV)-crosslinkable monomer, capable of forming a network of cross-linked chains. The EGMEA, bearing monovalent vinyl groups, is also UV-polymerizable, leading to a linear polymer with a high flexibility due to the presence of the oligomer ethylene oxide units. When the degree of cross-linking of ETPTA is moderate or low, the resulting ETPTA/EGMEA semi-IPN polymer provides good mechanical flexibility or elasticity and reasonable mechanical strength. The lithium-ion conductivity of this polymer is in the range from 10⁻⁴ to 5×10⁻³ S/cm.

The aforementioned high-elasticity polymers may be used alone to protect the lithium foil/coating layer at the anode. Alternatively, the high-elasticity polymer can be mixed with a broad array of elastomers, electrically conducting polymers, lithium ion-conducting materials, and/or strengthening materials (e.g. carbon nanotube, carbon nano-fiber, or graphene sheets).

A broad array of elastomers can be used alone as an elastic polymer or mixed with a high-elasticity polymer to form a blend, co-polymer, or interpenetrating network that encapsulates the cathode active material particles. The elastomeric material may be selected from natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer, and combinations thereof.

The urethane-urea copolymer film usually consists of two types of domains, soft domains and hard ones. Entangled linear backbone chains consisting of poly (tetramethylene ether) glycol (PTMEG) units constitute the soft domains, while repeated methylene diphenyl diisocyanate (MDI) and ethylene diamine (EDA) units constitute the hard domains. The lithium ion-conducting additive can be incorporated in the soft domains or other more amorphous zones.

In some embodiments, a high-elasticity polymer can form a polymer matrix composite containing a lithium ion-conducting additive dispersed in the high-elasticity polymer matrix material, wherein the lithium ion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.

In some embodiments, the high-elasticity polymer can be mixed with a lithium ion-conducting additive, which contains a lithium salt selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

In some embodiments, the high-elasticity polymer may form a mixture, co-polymer, or semi-interpenetrating network with a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivative thereof (e.g. sulfonated versions), or a combination thereof.

Unsaturated rubbers that can be used as an elastic polymer include natural polyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR for isoprene rubber), polybutadiene (BR for butadiene rubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene, IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer of styrene and butadiene, SBR), nitrile rubber (copolymer of butadiene and acrylonitrile, NBR),

Some elastomers are saturated rubbers that cannot be cured by sulfur vulcanization; they are made into a rubbery or elastomeric material via different means: e.g. by having a copolymer domain that holds other linear chains together. Each of these elastomers can be used to bond particles of a cathode active material by one of several means; e.g. spray coating, dilute solution mixing (dissolving the cathode active material particles in an uncured polymer, monomer, or oligomer, with or without an organic solvent) followed by drying and curing.

Saturated rubbers and related elastomers in this category include EPM (ethylene propylene rubber, a copolymer of ethylene and propylene), EPDM rubber (ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a diene-component), epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), protein resilin, and protein elastin. Polyurethane and its copolymers (e.g. urea-urethane copolymer) are particularly useful elastomeric shell materials for encapsulating anode active material particles.

The presently disclosed lithium secondary batteries can contain a wide variety of cathode active materials. The cathode active material layer may contain a cathode active material selected from an inorganic material, an organic material, a polymeric material, or a combination thereof. The inorganic material may be selected from a metal oxide, metal phosphate, metal silicide, metal selenide, transition metal sulfide, or a combination thereof.

The inorganic cathode active material may be selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, lithium metal silicide, or a combination thereof.

The cathode active material layer may contain a metal oxide containing vanadium oxide selected from the group consisting of Li_(x)VO₂, Li_(x)V₂O₅, Li_(x)V₃O₈, Li_(x)V₃O₇, Li_(x)V₄O₉, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metal phosphate, selected from a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.

The electrolyte used in the lithium battery may be a liquid electrolyte, polymer gel electrolyte, solid-state electrolyte (including solid polymer electrolyte, inorganic electrolyte, and composite electrolyte), quasi-solid electrolyte, ionic liquid electrolyte.

The liquid electrolyte or polymer gel electrolyte typically comprises a lithium salt dissolved in an organic solvent or ionic liquid solvent. There is no particular restriction on the types of lithium salt or solvent that can be used in practicing the present disclosures.

Some particularly useful lithium salts are lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.

There are a wide variety of processes that can be used to produce layers of elastic composite separators. These include coating, casting, painting, spraying (e.g. ultrasonic spraying), spray coating, printing (screen printing, 3D printing, etc.), tape casting, etc.

In certain embodiments, the process for manufacturing elastic composite separators may comprise (A) dispersing the flame retardant additive and optional particles of the inorganic solid electrolyte particles in a liquid reactive mass of an elastic polymer precursor to form a slurry; (B) dispensing and depositing a layer of the liquid reactive mass onto a solid substrate surface; and (C) polymerizing and/or curing the reactive mass to form a layer of elastic composite separator.

The solid substrate may be an anode current collector, an anode active material layer, or a cathode active material layer. In other words, this elastic composite separator can be directly deposited onto a layer of anode active material, an anode current collector, or a layer of cathode active material. This is achievable because curing of the high-elasticity polymer does not require a high temperature; curing temperature typically lower than 200° C. or even lower than 100° C. This is in stark contrast to the typically 900-1,200° C. required of sintering an inorganic solid electrolyte to form a ceramic separator. In addition, the presently disclosed elastic separator is at least as good as a ceramic separator in terms of reducing interfacial impedance and stopping dendrite penetration.

Preferably, the process is a roll-to-roll process wherein step (B) comprises (i) continuously feeding a layer of the solid substrate (e.g. flexible metal film, plastic film, etc.) from a feeder roller to a dispensing zone where the reactive mass is dispensed and deposited onto the solid substrate to form a continuous layer of the reactive mass; (ii) moving the layer of the reactive mass into a reacting zone where the reactive mass is exposed to heat, ultraviolet (UV) light, or high-energy radiation to polymerize and/or cure the reactive mass to form a continuous layer of elastic composite; and (iii) collecting the elastic composite on a winding roller. This process is conducted in a reel-to-reel manner.

In certain embodiments, as illustrated in FIG. 4, the roll-to-roll process may begin with continuously feeding a solid substrate layer 12 (e.g. PET film) from a feeder roller 10. A dispensing device 14 is operated to dispense and deposit a reactive mass 16 (e.g. slurry) onto the solid substrate layer 12, which is driven toward a pair of rollers (18 a, 18 b). These rollers are an example of a provision to regulate or control the thickness of the reactive mass 20. The reactive mass 20, supported on the solid substrate, is driven to move through a reacting zone 22 which is provided with a curing means (heat, UV, high energy radiation, etc.). The partially or fully cured polymer composite 24 is collected on a winding roller 26. One may unwind the roll at a later stage.

The process may further comprise cutting and trimming the layer of elastic composite into one or multiple pieces of elastic composite separators.

The process may further comprise combining an anode, the elastic composite separator, an electrolyte, and a cathode electrode to form a lithium battery.

The lithium battery may be a lithium metal battery, lithium-ion battery, lithium-sulfur battery, lithium-selenium battery, lithium-air battery, etc. The cathode active material in the lithium-sulfur battery may comprise sulfur or lithium polysulfide.

Example 1: Anode-Less Lithium Battery Containing an Elastic Composite Separator

The ethoxylated trimethylopropane triacrylate monomer (ETPTA, Mw=428, Sigma-Aldrich) was dissolved in a solvent mixture of ethylene carbonate (EC)/diethyl carbonate (DEC), at a weight-based composition ratio of the ETPTA/solvent of 3/97 (w/w). Subsequently, benzoyl peroxide (BPO, 1.0 wt. % relative to the ETPTA content), along with a desired amount of selected flame retardant (e.g. aluminum hydroxide and a phosphorus compound, Formula 3 below, from Amfine Chemical Corp.), were added as a radical initiator to allow for thermal crosslinking reaction upon deposition on a Cu foil surface. This layer of ETPTA monomer/initiator was then thermally cured at 60° C. for 30 min to obtain a protective layer.

On a separate basis, some amount of the ETPTA monomer/solvent/initiator solution was cast onto a glass surface to form a wet film, which was thermally dried and then cured at 60° C. for 30 min to form a film of cross-linked polymer. In this experiment, the BPO/ETPTA weight ratio was varied from 0.1% to 4% to vary the degree of cross-linking in several different polymer films. Some of the cured polymer samples were subjected to dynamic mechanical testing to obtain the equilibrium dynamic modulus, Ge, for the determination of the number average molecular weight between two cross-link points (Mc) and the corresponding number of repeat units (Nc), as a means of characterizing the degree of cross-linking. The typical and preferred number of repeat units (Nc) is from 5 to 5,000, more preferably from 10 to 1,000, further preferably from 20 to 500, and most preferably from 50 to 500.

Several tensile testing specimens were cut from each cross-link film and tested with a universal testing machine. Tensile testing results indicate that BPO-initiated cross-linked ETPTA polymers have an elastic deformation from approximately 230% to 700%. The above values are for neat polymers without any additive. The addition of up to 30% by weight of an additive typically reduces this elasticity down to a reversible tensile strain in the range from 10% to 120%.

For electrochemical testing, the working electrodes (cathode layers) were prepared by mixing 85 wt. % LiV₂O₅ or 88% of graphene-embraced LiV₂O₅ particles, 5-8 wt. % CNTs, and 7 wt. % polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5 wt. % total solid content. After coating the slurries on Al foil, the electrodes were dried at 120° C. in vacuum for 2 h to remove the solvent before pressing. Then, the electrodes were cut into a disk (ϕ=12 mm) and dried at 100° C. for 24 h in vacuum.

Electrochemical measurements were carried out using CR2032 (3V) coin-type cells with lithium metal as the counter/reference electrode, an elastic composite separator, and 1 M LiPF₆ electrolyte solution dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). The cell assembly was performed in an argon-filled glove-box. For comparison purposes, cells with the conventional Celgard 2400 membrane as separator were also prepared. The CV measurements were carried out using a CH-6 electrochemical workstation at a scanning rate of 1-100 mV/s. The electrochemical performance of the cell featuring the elastic composite separator and that containing a conventional separator were evaluated by galvanostatic charge/discharge cycling at a current density of 50 mA/g using an Arbin electrochemical workstation.

The specific intercalation capacity curves of two lithium cells each having a cathode containing LiV₂O₅ particles (one cell having a retardant-filled cross-linked ETPTA polymer-based separator and the other having no protection, just a conventional separator) were obtained and compared. As the number of cycles increases, the specific capacity of the un-protected cells drops at a very fast rate. In contrast, the presently disclosed cross-linked ETPTA polymer-based composite separator (also serving as a protection layer) provides the battery cell with a significantly more stable and high specific capacity for a large number of cycles. These data have demonstrated the surprising and superior performance of the presently disclosed cross-linked ETPTA polymer protection approach.

The high-elasticity cross-linked ETPTA composite separator appears to be capable of reversibly deforming to a great extent without breakage when the lithium foil decreases in thickness during battery discharge. The elastic composite separator also prevents the continued reaction between liquid electrolyte and lithium metal at the anode, reducing the problem of continuing loss in lithium and electrolyte. This also enables a significantly more uniform deposition of lithium ions upon returning from the cathode during a battery re-charge; hence, no lithium dendrite.

Example 2: High-Elasticity Polymer Composite Layer Implemented as a Composite Separator in a Lithium-LiCoO₂ Cell (Initially the Cell being Lithium-Free)

The high-elasticity polymer use in an elastic composite separator was based on cationic polymerization and cross-linking of the cyanoethyl polyvinyl alcohol (PVA-CN) in succinonitrile (SN). The procedure began with dissolving PVA-CN in succinonitrile to form a mixture solution. This step was followed by adding an initiator into the solution. For the purpose of incorporating some lithium species into the high elasticity polymer, we chose to use LiPF₆ as an initiator. The ratio between LiPF₆ and the PVA-CN/SN mixture solution was varied from 1/20 to 1/2 by weight to form a series of precursor solutions. A desired amount of a flame retardant (e.g. decabromodiphenyl ethane (DBDPE), brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO), and melamine-based flame retardant, separately; the latter from Italmatch Chemicals) was then added into the solution. In several samples, a garnet-type solid electrolyte (Li₇La₃Zr₂O₁₂ (LLZO) powder) was also added into the solution. Subsequently, these solutions were separately spray-deposited to form a thin layer of precursor reactive mass onto a Cu foil. The precursor reactive mass was then heated at a temperature from 75 to 100° C. for 2 to 8 hours to obtain a layer of high-elasticity polymer composite adhered to the Cu foil surface.

Additionally, some amount of the reacting mass, PVA-CN/LiPF₆, was cast onto a glass surface to form several films which were polymerized and cross-linked to obtain cross-linked polymers having different degrees of cross-linking. Tensile testing was also conducted on these films and this series of cross-linked polymers can be elastically stretched up to approximately 80% (higher degree of cross-linking) to 400% (lower degree of cross-linking).

Electrochemical testing results show that the cell having an anode-protecting polymer composite layer offers a significantly more stable cycling behavior. The high-elasticity polymer appears to act to isolate the liquid electrolyte from the subsequently deposited lithium coating, preventing continued reaction between the liquid electrolyte and lithium metal.

Example 3: Li Metal Cells Containing a PETEA-Based Polymer Composite Separator

For preparing as an anode lithium metal-protecting layer, pentaerythritol tetraacrylate (PETEA), Formula 4, was used as a monomer:

In a representative procedure, the precursor solution was composed of 1.5 wt. % PETEA (C₁₇H₂₀O₈) monomer and 0.1 wt. % azodiisobutyronitrile (AIBN, C₈H₁₂N₄) initiator dissolved in a solvent mixture of 1,2-dioxolane (DOL)/dimethoxymethane (DME) (1:1 by volume). The PETEA/AIBN precursor solution, along with a flame retardant (e.g. triphenylphosphate (TPP) and “Phoslite”, a phosphorus compound from Italmatch Chemicals) dispersed therein, was cast onto a lithium metal layer pre-deposited on a Cu foil surface to form a precursor film, which was polymerized and cured at 70° C. for half an hour to obtain a lightly cross-linked polymer.

Additionally, the reacting mass, PETEA/AIBN (without any additive), was cast onto a glass surface to form several films that were polymerized and cured to obtain cross-linked polymers having different degrees of cross-linking. Tensile testing was also conducted on these films and this series of cross-linked polymers can be elastically stretched up to approximately 25% (higher degree of cross-linking) to 80% (lower degree of cross-linking)

Commercially available NCM-532 powder (well-known lithium nickel cobalt manganese oxide), along with graphene sheets (as a conductive additive), was then added into an NMP and PVDF binder suspension to form a multiple-component slurry. The slurry was then slurry-coated on Al foil and dried to form cathode layers.

An Al foil-supported cathode active layer, an elastic flame retardant composite separator, and a Cu foil-supported lithium metal foil are then assembled into a lithium metal cell, followed by an electrolyte injection procedure.

Example 4: Li Metal Cells Containing a Sulfonated Triblock Copolymer Poly(styrene-isobutylene-styrene, or SIBS) Composite as an Elastic Composite Separator

Both non-sulfonated and sulfonated elastomer composites were used to build an anode-protecting layer in the anode-less lithium cells. The sulfonated versions typically provide a much higher lithium ion conductivity and, hence, enable higher-rate capability or higher power density. The elastomer matrix can contain a lithium ion-conducting additive, in addition to the flame retardant, if so desired.

An example of the sulfonation procedure used in this study for making a sulfonated elastomer is summarized as follows: a 10% (w/v) solution of SIBS (50 g) and a desired amount of an inorganic filler material (0 to 40.5% by wt.) in methylene chloride (500 ml) was prepared. The solution was stirred and refluxed at approximately 40° C., while a specified amount of acetyl sulfate in methylene chloride was slowly added to begin the sulfonation reaction. Acetyl sulfate in methylene chloride was prepared prior to this reaction by cooling 150 ml of methylene chloride in an ice bath for approximately 10 min. A specified amount of acetic anhydride and sulfuric acid was then added to the chilled methylene chloride under stirring conditions. Sulfuric acid was added approximately 10 min after the addition of acetic anhydride with acetic anhydride in excess of a 1:1 mole ratio. This solution was then allowed to return to room temperature before addition to the reaction vessel.

After approximately 5 h, the reaction was terminated by slowly adding 100 ml of methanol. The reacted polymer solution was then precipitated with deionized water. The precipitate was washed several times with water and methanol, separately, and then dried in a vacuum oven at 50° C. for 24 h. This washing and drying procedure was repeated until the pH of the wash water was neutral. After this process, the final polymer yield was approximately 98% on average. This sulfonation procedure was repeated with different amounts of acetyl sulfate to produce several sulfonated polymers with various levels of sulfonation or ion-exchange capacities (IECs). The mol % sulfonation is defined as: mol %=(moles of sulfonic acid/moles of styrene)×100%, and the IEC is defined as the mille-equivalents of sulfonic acid per gram of polymer (mequiv./g).

After sulfonation and washing of each polymer, the S-SIBS samples were dissolved in a mixed solvent of toluene/hexanol (85/15, w/w) with concentrations ranging from 0.5 to 2.5% (w/v). Desired amounts of a flame retardant (aluminum hydroxide and magnesium hydroxide), along with some desired particles of a solid inorganic electrolyte, were then added into the solution to form slurry samples. The slurry samples were slot-die coated on a PET plastic substrate to form layers of sulfonated elastomer composite. These flame retardants were found to have insignificant impact on the lithium ion conductivity.

Example 5: Elastic Polyurethane Elastomer Containing Particles of a Flame Retardant Additive

Twenty four parts by weight of diphenylmethane diisocyanate and 22 parts by weight of butylene glycol were continuously reacted with 100 parts by weight of polyethylene adipate having hydroxyl groups at both terminals (molecular weight of 2,100) with agitation at a reaction temperature of 115° C. for a reaction time of 60 minutes to give a prepolymer having hydroxyl-terminal. This prepolymer having hydroxyl-terminal had a viscosity of 4,000 cP at 70° C.

On a separate basis, 84 parts by weight of diphenylmethane diisocyanate was continuously reacted with 200 parts by weight of polyethylene adipate having hydroxyl groups at both terminals (molecular weight of 2,100) with agitation at a reaction temperature of 115° C. for a reaction time of 60 minutes to give a prepolymer having isocyanate-terminal. This prepolymer having isocyanate-terminal had a viscosity of 1,500 cP at 70° C.

One hundred forty six (146) parts by weight of the thus obtained prepolymer having hydroxyl-terminal and 284 parts by weight of the obtained prepolymer having isocyanate-terminal, along with a desired amount of a selected flame retardant additive (separately aluminum hydroxide reactive and polysiloxane; the latter from Evonik), were continuously injected into a heat exchange reactor and mixed and stirred at a reaction temperature of 190° C. for a residence time of 5-30 minutes. The obtained viscous product was immediately cast onto a glass surface to obtain a layer of elastic polymer composite having a thickness of approximately 51 nm, 505 nm, and 2.2 μm, respectively.

On a separate basis, a sample of reactive polysiloxane was cast onto a glass surface and cured at 115° C. for 2 hours to form elastic polymer composite film of 8-45 μm in thickness. The lithium ion conductivity of these thin films was approximately 2-5 10⁻⁵ S/cm.

Example 6: Preparation of Solid Electrolyte Powder, Lithium Nitride Phosphate Compound (LIPON)

Particles of Li₃PO₄ (average particle size 4 μm) and urea were prepared as raw materials; 5 g each of Li₃PO₄ and urea was weighed and mixed in a mortar to obtain a raw material composition. Subsequently, the raw material composition was molded into 1 cm×1 cm×10 cm rod with a molding machine, and the obtained rod was put into a glass tube and evacuated. The glass tube was then subjected to heating at 500° C. for 3 hours in a tubular furnace to obtain a lithium nitride phosphate compound (LIPON). The compound was ground in a mortar into a powder form. These particles can be added into a polymer composite matrix.

Example 7: Preparation of Solid Electrolyte Powder, Lithium Superionic Conductors with the

Li₁₀GeP₂S₁₂ (LGPS)-Type Structure

The starting materials, Li₂S and SiO₂ powders, were milled to obtain fine particles using a ball-milling apparatus. These starting materials were then mixed together with P₂S₅ in the appropriate molar ratios in an Ar-filled glove box. The mixture was then placed in a stainless steel pot, and milled for 90 min using a high-intensity ball mill. The specimens were then pressed into pellets, placed into a graphite crucible, and then sealed at 10 Pa in a carbon-coated quartz tube. After being heated at a reaction temperature of 1,000° C. for 5 h, the tube was quenched into ice water. The resulting solid electrolyte material was then subjected to grinding in a mortar to form a powder sample to be later added as an inorganic solid electrolyte particles dispersed in an intended elastic polymer matrix.

Example 8: Preparation of Garnet-Type Solid Electrolyte Powder

The synthesis of the c-Li_(6.25)Al_(0.25)La₃Zr₂O₁₂ was based on a modified sol-gel synthesis-combustion method, resulting in sub-micron-sized particles after calcination at a temperature of 650° C. (J. van den Broek, S. Afyon and J. L. M. Rupp, Adv. Energy Mater., 2016, 6, 1600736).

For the synthesis of cubic garnet particles of the composition c-Li_(6.25)Al_(0.25)La₃Zr₂O₁₂, stoichiometric amounts of LiNO₃, Al(NO₃)₃-9H₂O, La(NO₃)₃-6(H₂O), and zirconium (IV) acetylacetonate were dissolved in a water/ethanol mixture at temperatures of 70° C. To avoid possible Li-loss during calcination and sintering, the lithium precursor was taken in a slight excess of 10 wt % relative to the other precursors. The solvent was left to evaporate overnight at 95° C. to obtain a dry xerogel, which was ground in a mortar and calcined in a vertical tube furnace at 650° C. for 15 h in alumina crucibles under a constant synthetic airflow. Calcination directly yielded the cubic phase c-Li_(6.25)Al_(0.25)La₃Zr₂O₁₂, which was ground to a fine powder in a mortar for further processing.

The c-Li_(6.25)Al_(0.25)La₃Zr₂O₁₂ solid electrolyte pellets with relative densities of ˜87±3% made from this powder (sintered in a horizontal tube furnace at 1070° C. for 10 h under O₂ atmosphere) exhibited an ionic conductivity of ˜0.5×10⁻³ S cm⁻¹ (RT). The garnet-type solid electrolyte with a composition of c-Li_(6.25)Al_(0.25)La₃Zr₂O₁₂ (LLZO) in a powder form was dispersed in several electric, ion-conducting polymers discussed earlier.

Example 9: Preparation of Sodium Superionic Conductor (NASICON) Type Solid Electrolyte Powder

The Na_(3.1)Zr_(1.95)M_(0.05)Si₂PO₁₂ (M=Mg, Ca, Sr, Ba) materials were synthesized by doping with alkaline earth ions at octahedral 6-coordination Zr sites. The procedure employed includes two sequential steps. Firstly, solid solutions of alkaline earth metal oxides (MO) and ZrO₂ were synthesized by high energy ball milling at 875 rpm for 2 h. Then NASICON Zr_(1.95)M_(0.05)Si₂PO₁₂ structures were synthesized through solid-state reaction of Na₂CO₃, Zr_(1.95)M_(0.05)O_(3.95), SiO₂, and NH₄H₂PO₄ at 1260° C. 

We claim:
 1. A lithium secondary battery comprising a cathode, an anode, an elastic and flame-retardant composite separator disposed between the cathode and the anode, and a working electrolyte through which lithium ions are transported between the anode and the cathode during a battery charge or discharge, wherein said elastic and flame-retardant composite separator comprises a high-elasticity polymer and from 0.1% to 95% by weight of a flame retardant additive dispersed in, dissolved in, or chemically bonded to the high-elasticity polymer, wherein said elastic composite separator has a thickness from 50 nm to 100 μm and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm at room temperature and said high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% when measured without any additive dispersed therein.
 2. The lithium secondary battery of claim 1, wherein the battery is a lithium metal battery and the anode has an anode current collector but initially the anode has no lithium or lithium alloy as an anode active material supported by said anode current collector when the battery is made and prior to a charge or discharge operation of the battery.
 3. The lithium secondary battery of claim 1, wherein the battery is a lithium metal battery and the anode has an anode current collector and an amount of lithium or lithium alloy as an anode active material supported by said anode current collector.
 4. The lithium secondary battery of claim 1, wherein the battery is a lithium-ion battery and the anode has an anode current collector and a layer of an anode active material supported by said anode current collector, wherein the anode active materials is selected from the group consisting of: (a) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), phosphorus (P), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, V, or Cd, and their mixtures, composites, or lithium-containing composites; (d) salts and hydroxides of Sn; (e) lithium titanate, lithium manganate, lithium aluminate, lithium titanium niobium oxide, lithium-containing titanium oxide, lithium transition metal oxide, ZnCo₂O₄; (f) carbon or graphite particles (g) prelithiated versions thereof; and (h) combinations thereof.
 5. The lithium secondary battery of claim 1, wherein said flame retardant additive is selected from a halogenated flame retardant, phosphorus-based flame retardant, melamine flame retardant, metal hydroxide flame retardant, silicon-based flame retardant, phosphate flame retardant, biomolecular flame retardant, or a combination thereof.
 6. The lithium secondary battery of claim 1, wherein said flame retardant additive is in a form of encapsulated particles comprising the additive encapsulated by a shell of a substantially lithium ion-impermeable and liquid electrolyte-impermeable coating material, wherein said shell is breakable when exposed to a temperature higher than a threshold temperature.
 7. The lithium secondary battery of claim 1, wherein said working electrolyte is selected from an organic liquid electrolyte, ionic liquid electrolyte, polymer gel electrolyte, solid-state electrolyte, quasi-solid electrolyte having a lithium salt dissolved in an organic or ionic liquid with a lithium salt concentration higher than 2.0 M, hybrid or composite electrolyte, or a combination thereof.
 8. The lithium secondary battery of claim 1, wherein said elastic and flame-retardant composite separator further comprises 1%-90% by weight of particles of an inorganic solid electrolyte material selected from an oxide type, sulfide type, hydride type, halide type, borate type, phosphate type, lithium phosphorus oxynitride (UPON), garnet-type, lithium superionic conductor (LISICON) type, sodium superionic conductor (NASICON) type, or a combination thereof.
 9. The lithium secondary battery of claim 1, wherein said high-elasticity polymer contains a cross-linked network of polymer chains having an ether linkage, nitrile-derived linkage, benzo peroxide-derived linkage, ethylene oxide linkage, propylene oxide linkage, vinyl alcohol linkage, cyano-resin linkage, triacrylate monomer-derived linkage, tetraacrylate monomer-derived linkage, or a combination thereof in said cross-linked network of polymer chains.
 10. The lithium secondary battery of claim 1, wherein said elastic polymer contains a cross-linked network of polymer chains selected from nitrile-containing polyvinyl alcohol chains, cyanoresin chains, pentaerythritol tetraacrylate chains, pentaerythritol triacrylate chains, ethoxylated trimethylolpropane triacrylate (ETPTA) chains, ethylene glycol methyl ether acrylate (EGMEA) chains, acrylic acid-derived chains, polyvinyl alcohol chains, or a combination thereof.
 11. The lithium secondary battery of claim 1, wherein said elastic polymer comprises an elastomer selected from natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butyl rubber, styrene-butadiene rubber, nitrile rubber, ethylene propylene rubber, ethylene propylene diene rubber, metallocene-based poly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer, styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplastic elastomer, protein resilin, protein elastin, ethylene oxide-epichlorohydrin copolymer, polysiloxane, polyurethane, urethane-urea copolymer, urethane-acrylic copolymer, a copolymer thereof, a sulfonated version thereof, or a combination thereof.
 12. The lithium secondary battery of claim 1, wherein said elastic polymer further comprises from 0.1% to 30% by weight of a lithium ion-conducting additive, which is different from the inorganic solid electrolyte particles in composition or structure.
 13. The lithium secondary battery of claim 12, wherein said lithium ion-conducting additive comprises a lithium salt selected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.
 14. The lithium secondary battery of claim 12, wherein said lithium ion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.
 15. The lithium secondary battery of claim 1, wherein the high-elasticity polymer in the composite separator forms a mixture or blend with a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof.
 16. The lithium secondary battery of claim 1, wherein said cathode comprises a cathode active material selected from an inorganic material, an organic material, a polymeric material, or a combination thereof.
 17. The lithium secondary battery of claim 16, wherein said inorganic material, as a cathode active material, is selected from sulfur, selenium, a metal oxide, metal phosphate, metal silicide, metal selenide, metal sulfide, or a combination thereof.
 18. The lithium secondary battery of claim 16, wherein said inorganic material is selected from a lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphate, lithium metal silicide, or a combination thereof.
 19. The lithium secondary battery of claim 16, wherein said inorganic material is selected from a lithium transition metal silicate, denoted as Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄, wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb is selected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y≤1.
 20. The lithium secondary battery of claim 17, wherein said metal oxide contains a vanadium oxide selected from the group consisting of Li_(x)VO₂, Li_(x)V₂O₅, Li_(x)V₃O₈, Li_(x)V₃O₇, Li_(x)V₄O₉, Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5.
 21. The lithium secondary battery of claim 17, wherein said metal oxide or metal phosphate is selected from a layered compound LiMO₂, spinel compound LiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavorite compound LiMPO₄F, borate compound LiMBO₃, or a combination thereof, wherein M is a transition metal or a mixture of multiple transition metals.
 22. The lithium secondary battery of claim 1, wherein said elastic flame-retardant composite separator has a gradient concentration of the flame retardant additive.
 23. A process for manufacturing the elastic flame-retardant composite separator of claim 1, the process comprising (A) dispersing particles of said flame retardant additive, optionally along with an inorganic solid electrolyte particles, in a liquid reactive mass of an elastic polymer precursor to form a slurry; (B) dispensing and depositing a layer of said liquid reactive mass onto a solid substrate surface; and (C) polymerizing and/or curing said reactive mass to form said layer of elastic flame-retardant composite separator.
 24. The process of claim 23, wherein said solid substrate is an anode current collector, an anode active material layer, or a cathode active material layer.
 25. The process of claim 23, which is a roll-to-roll process wherein said step (B) comprises (i) continuously feeding a layer of said solid substrate from a feeder roller to a dispensing zone where said reactive mass is dispensed and deposited onto said solid substrate to form a continuous layer of said reactive mass; (ii) moving said layer of the reactive mass into a reacting zone where the reactive mass is exposed to heat, ultraviolet light, or high-energy radiation to polymerize and/or crosslink said reactive mass to form a continuous layer or roll of elastic flame-retardant composite; and (iii) collecting said elastic flame-retardant composite on a winding roller.
 26. The process of claim 25, further comprising cutting and trimming said layer or roll of elastic flame-retardant composite into one or multiple pieces of elastic flame-retardant composite layers.
 27. The process of claim 23, further comprising a step of combining an anode, said elastic flame-retardant composite separator, a working electrolyte, and a cathode electrode to form a lithium battery.
 28. An elastic and flame retardant composite layer, wherein said elastic and flame retardant composite comprises a high-elasticity polymer and from 0.1% to 95% by weight of a flame retardant additive dispersed in, dissolved in, or chemically bonded to the high-elasticity polymer, wherein said elastic composite separator has a thickness from 50 nm to 100 μm and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm at room temperature and said high elasticity polymer has a fully recoverable tensile strain from 2% to 1,000% when measured without any additive dispersed therein. 